[蛋白质.合集].NMR--Essential.Practical.NMR.anic.Chemistry.pdf

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Getting Started
The Technique
This book is not really intended to give an in-depth education in all aspects of the NMR effect (there are
numerous excellent texts if you want more information) but we will try to deal with some of the more
pertinent ones.
The first thing to understand about NMR is just how insensitive it pared with many other
analytical techniques. This is because of the origin of the NMR signal itself.
The NMR signal arises from a quantum mechanical property of nuclei called ‘spin’. In the text
here, we will use the example of the hydrogen nucleus (proton) as this is the nucleus that we will be
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dealing with mostly. Protons have a ‘spin quantum number’ of /2. In this case, when they are placed
in a ic field, there are two possible spin states that the nucleus can adopt and there is an energy
difference between them (Figure ).
The energy difference between these levels is very small, which means that the population difference
is also small. The NMR signal arises from this population difference and hence the signal is also very
small. There are several factors which influence the population difference and these include the nature
of the nucleus (its ‘ic ratio’) and the strength of the ic field that they are placed in.
The equation that relates these factors (and the only one in this book) is shown here:
γ=
γ hB ic ratio

E = h = Planck’s constant
π
2 B = ic field strength
Because the sensitivity of the technique goes up with ic field, there has been a drive to increase
the strength of the s to improve sensitivity.
Unfortunately, this improvement has been linear since the first NMR s (with a few kinks here
and there). This means that in percentage terms, the benefits have e smaller as development has
continued. But sensitivity has not been the only factor driving the search for more powerful s.
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